If ARMS were to get a second fighter, it would most likely depend on the direction that the series takes.
If ARMS 2 did what literally every other fighting game is doing and is including a big cinematic story mode as a major selling point, whoever the protagonist of that is will likely be the next fighter in Smash. I imagine that would most likely be Spring Man, not only because he's the series' mascot, but he has already has a relatively developed backstory (up and comer who inherited a title that has been used by two previous fighters in the past) and he is implied to have connections to Max Brass and possibly Dr. Coyle, if Springtron is anything to go by.
If ARMS 2 follows the ensemble cast rule and technically doesn't have a main character, it could really be whoever Sakurai and Yabuki think would be the most interesting. Given how he was the runner-up, I'd assume Ninjara would be used for this role.